I like this. I didn’t know about context-sensitive in regex.

It seemed the regex engine was Oniguruma/ruby.

$ nc 2171
Hi! We want to hire some TRUE regular expression hackers to write firewall rules.
So here are five interview questions for you, good luck!
Note: After CVE-2015-4410, we reject everything contains newline, so you can just safely use ^ and $ here.

================= [SQL] =================
Please match string that contains "select" as a case insensitive subsequence.
Running on test #885...Accepted :)

=============== [a^nb^n] ================
Yes, we know it is a classical example of context free grammer.
Running on test #370...Accepted :)

================= [x^p] =================
A prime is a natural number greater than 1 that has no positive divisors other than 1 and itself.
Running on test #304...Accepted :)

============= [Palindrome] ==============
Both "QQ" and "TAT" are palindromes, but "PPAP" is not.
Running on test #799...Accepted :)

============== [a^nb^nc^n] ==============
Is CFG too easy for you? How about some context SENSITIVE grammer?
Running on test #504...Accepted :)

Congratz! Here is your singing bonus:
hitcon{The pumping lemma is a lie, just like the cake}